1. Field of the Invention
This invention relates to an apparatus for fine adjustment of position of an object and more particularly to stepwise displacement of an object of a very minute unit length.
2. Description of the Prior Art
Fine positioning adjustment to an accuracy below one micron is often needed in the field of accurate devices. Particularly, specimens should be moved to an extremely high accuracy in observation, measurement and treatment instruments using X-rays or charged particles.
Thus, various types of fine movement devices have been used, but they use purely mechanical structures utilizing screws, cams, levers, etc. Larger devices are needed for achieving higher accuracy. Further, it is impossible to completely remove minute errors due to structural coupling of mechanical elements.
An example of a specimen positioning mechanism used in conventional electron-microscopes is shown in FIG. 1, where rotation of a knob 11 is transformed into linear motion and pushes a specimen substrate 1 through a pin 12. The knob 11 is fixed on one end of a transmission rod 9. The transmission rod 9 is supported by a holder 10 and has a fine movement screw portion 4 at the other end. The screw portion 4 transforms rotating motion into linear motion. The linear motion is transmitted through a construction lever 3, and a transmission rod 2 supported by a joint 5 and a push pin 12, to the specimen substrate 1. The specimen substrate 1 supporting a specimen cylinder 6 is positioned at a rest position using push-springs 13 and pull-springs 8. Fine movement is given to such a specimen substrate using said mechanism to finely adjust the position of the specimen with respect to an electron lens 7.
This conventional fine movement device of purely mechanical structure has, however, a large drawback that since every mechanical joint portion is accompanied with a tolerance, such tolerances are successively multiplied from the knob 11 to the specimen substrate 1 to cause a large positioning error. Further, each mechanical joint portion is positioned only by spring force. When external vibration is transmitted to or thermal expansion is caused in the mechanical elements, the positioning accuracy is inevitably much decreased.